Kosh,
If you bought it as a 4GB player the problem could be memory chip related.
At present these players only support a maximum of 2GB but some manufacturers hack the chips so that they read much larger than their true size. In some cases 4GB chips have in fact been hacked 128MB chips.
Hacked players tend to run fine for a while but as you start loading more and more files or add and remove files they start playing up, generally with file corruption and player restarts.
Once you start having problems you need to format them which will remove the hack and your players memory will then read it's true size.
They can however be difficult to format so if you have trouble try this method
HERE.
If you have tried different earphones and and the audio problem still exists it may be a lose connection in the players audio jack so you may need to open the player to check that. If you do open the player you can verify the memory chip size against
THIS list of Samsung and Hynix chips.